The present invention relates to a clock recovery circuit for reproducing a sampling clock signal from a PSK demodulated signal to sample the PSK demodulated signal at its eye aperture point.
A prior clock recovery method which reproduces a sampling clock signal from a PSK demodulated signal through a zero cross technique has been known. It should be appreciated that a zero cross point refers to a point where adjoining reception points of the PSK demodulated signal intersect an I-axis or a Q-axis.
The PSK demodulated signal is limited in band by filtering or any other technique and in particular, a QPSK demodulated signal has a constellation as shown in FIG. 9(a) and a dull waveform as shown in FIG. 9(b). To obtain correct mapping information from the PSK demodulated signal with such a dull waveform, there exists the need for reproducing a clock signal to sample the PSK demodulated signal at a point called an eye aperture point.
A timing for this sampling is determined through the zero cross technique which uses a point where adjoining reception points of the PSK demodulated signal intersect the I-axis or the Q-axis. Namely, the eye aperture point is determined based on the zero cross point. This is due to the fact that there exists a point where the amplitude of the PSK demodulated signal is zero, that is, where the PSK demodulated signal intersects the I-axis or the Q-axis.
For example, if a reception point transits from the point A to the point B as shown in FIG. 10(a), there may exist a slight deviation in time between the midpoint C in time for eye aperture points and the zero cross point D as shown in FIG. 10(b). A clock signal can be extracted from a received signal by detecting this deviation and correcting the sampling timing in a direction to eliminate this deviation. This is the clock recovery method through an ordinary zero cross technique.
It should be appreciated that a QPSK demodulated signal with no reception point on the I-axis or the Q-axis is defined to have its adjoining reception points intersect the I-axis or the Q-axis. Therefore, for the QPSK demodulated signal, even if a zero cross point deviates from the midpoint in time for eye aperture points, a correct timing deviation could be calculated.
However, since an 8PSK demodulated signal has its reception points on the I-axis or the Q-axis as shown in the constellation of FIG. 11(a) and the waveform of FIG. 11(b), a point which is not an actual zero cross point may be detected as a zero cross point, that is, an intersection point on the I-axis may be wrongly recognized as a zero cross point, for example, when a reception point on the I-axis follows a reception point in the third quadrant and the reception point on the I-axis deviates in the positive direction of the Q-axis from some reason.
Thus, in the prior clock signal reproduction method, reception points during the interval xcex1 as shown in FIG. 11 (b) are not used for zero cross detection and are cancelled so that reception points demodulated in the proximity of the I-axis or the Q-axis can be recognized as those on the axis to avoid any significant error in detecting zero cross points and to allow for sampling at eye aperture points of the 8PSK demodulated signal.
However, a prior clock recovery circuit cannot ensure accurate detection of zero cross points because it does not determine whether a point of interest is a reception point in the proximity of the I-axis or the Q-axis. In addition, the 8PSK demodulated signal has a maximum value, a minimum value, and a value on the I-axis, as well as a midpoint value, and thus, if a reception point transits from the point A to the point B as shown in the constellation of FIG. 12(a), the midpoint C in time for adjoining eye aperture points may deviate from a detected zero cross point D as shown in FIG. 12(b) and there may exist a phase error due to a deviation between the midpoint C and the zero cross point D.
Furthermore, in the prior circuit, zero cross detection is performed by using an absolute phase technique which allows a signal at the receiver to be in phase with a signal at the transmitter and thus, a reception point may exist on the axis and fewer zero cross points can be detected.
It is an object of the present invention to provide a clock recovery circuit which can reproduce a clock signal accurately from a PSK demodulated signal in order to sample an eye aperture point.
A clock recovery circuit according to the present invention includes: a phase rotation circuit for phase-rotating a PSK demodulated signal by an angle predetermined from a transition angle between adjoining reception points; and a phase error detection circuit for detecting a phase error due to a time difference between a zero cross point where the adjoining reception points of the PSK demodulated signal phase-rotated by the phase rotation circuit intersect the I-axis or the Q-axis and the midpoint in time for the adjoining reception points, so that a sampling location for the adjoining reception points of the PSK demodulated signal is corrected based on the phase error detected by the phase error detection circuit in order to allow for sampling at an eye aperture point.
With the clock recovery circuit according to the present invention, the PSK demodulated signal is phase-rotated by the phase rotation circuit by the angle predetermined from the transition angle between adjoining reception points and the phase error due to the time difference between the zero cross point where the adjoining reception points of the PSK demodulated signal phase-rotated by the phase rotation circuit intersect the I-axis or the Q-axis and the midpoint in time for the adjoining reception points is detected by the phase error detection circuit, so that the sampling location for the adjoining reception points of the PSK demodulated signal is corrected based on the phase error detected by the phase error detection circuit in order to allow for sampling at an eye aperture point.
Accordingly, a sampling point obtained based on a clock reproduced by the clock recovery circuit according to the present invention can coincide with the eye aperture point to allow for sampling at the eye aperture point.